1. Field of the Invention
The present invention relates to an actuator equipped with a drive mechanism, a flexible element interposed between an output shaft of the drive mechanism and a load, and a controller which controls an output to be applied to the load from the drive mechanism through the intermediary of the flexible element by controlling the operation of the drive mechanism, and a robot.
2. Description of the Related Art
There are the following two techniques, as engineering practices, for imparting compliance or flexibility to an arm of a robot so as to protect the arm or a component thereof from damage if the arm is subjected to an external force.
According to a first technique, an external force applied to the arm is measured on the basis of an output signal from an external force sensor provided in the arm, and a measurement result is input to a compliance model to obtain an output. Based on a command obtained as the result of the output, the joint angle or the joint displacement of the arm is controlled.
According to a second technique, a system for transmitting a driving force from a motor to a link constituting the arm is equipped with a flexible element in order to have the flexible element make up for the compliance in a frequency range in which the motor is unresponsive. As the second technique, there has been proposed a technique in which the expansion/contraction of the flexible element follows a torque command value, so that the expansion/contraction of the flexible element is not influenced by the joint displacement or the velocity of the arm, thus achieving a torque actuator which faithfully follows the torque command value (refer to Japanese Patent Application Laid-Open No. 2005-349555).
However, according to the first technique, improving the compliance requires prompt responses of the arm to the command. This in turn requires high rigidity of the components of the arm of the link or the like and prompt responsiveness of the motor. In particular, there is a restriction that the rigidity of the arm cannot be reduced because of the need for securing prompt motions of the arm in response to commands, thus making it difficult to achieve a lighter-weight, compact arm and to eventually achieve a multifunctional arm.
Further, according to the second technique, in order to add a damping element to a control system, it is necessary to provide a velocity feedback loop outside an expansion/contraction control system feedback loop. Therefore, in a control block diagram, the expansion/contraction control system feedback loop is disposed as a minor loop of the velocity feedback loop, leading to large orders of an object to be controlled and the control system. This results in deteriorated allowances of phases and gains, making it difficult to fully exercise damping effect.